Structural and Vibrational Properties of Iodopentafluorobenzene: A Combined Raman and Infrared Spectral and Theoretical Study

An accurate vibrational signature in halogen bonded molecular crystals

The far infrared (FIR) and Raman fingerprints of the halogen bond in two representative 1D halogen bonded networks based on the recognition of TFIB, tetrafluorodiiodobenzene, with piperazine or azopyridine, have been accurately identified. It was demonstrated that the signature of the halogen bonding in the solid state, especially the N⋯I signal can be simply and directly evidenced in the far infrared region. The DFT theoretical calculations identified the N⋯I interaction in the molecular crystals and allowed estimation of the corresponding energies and distances of the involved halogen bonds, in accordance with the cristallographic data.

How to treat C-F stretching vibrations? A vibrational CD study on chiral fluorinated molecules

The analysis of vibrational circular dichroism and infrared spectra typically requires the prediction of spectra on the density functional theory level. In particular for absolute configuration determinations, for which the comparison between experiment and theory is often supported by similarity analysis algorithms, it is important that frequencies, relative band intensities and VCD signs are predicted correctly. Due to the poor prediction of harmonic frequencies, carbon-fluorine stretching vibrations are often strongly misplaced by common hybrid functionals such as B3LYP. Herein we show that the M06-2X functional provides harmonic C-F stretching frequencies with an accuracy sufficient for a reliable spectra analysis. We briefly discuss the origin of this exceptional performance and show that it is likely to be related to a cancellation of errors.